The present invention relates to lumber stacking methods and apparatus.
It is conventional with known lumber stacking methods and apparatus to convey a layer of lumber which is to be situated at the top of a stack which is being formed to a lifting location situated adjacent but in advance of the stacking location, with this layer at the lifting location then being raised and transported to a location over the stack which is being formed, whereupon the transported layer is lowered to be deposited on top of the stack, forming at least temporarily the uppermost layer thereof. The arms are retracted back from the stacking location to the lifting location in preparation for transporting the next layer which is conveyed to the lifting location to the stacking location.
A conventional method and apparatus of the above type is shown, for example, in Finnish Pat. No. 42,188. However, with such a conventional method and apparatus, the lifting arms have at their upper edge regions which engage the layer of lumber at the underside thereof teeth which provide between the arms and the lifted layer of lumber a friction sufficient to prevent movement of the pieces of lumber with respect to the arms while the latter transports the layer of lumber from the lifting location to the stacking location. However such teeth provide undesirable marks in the pieces of lumber, and such marks are unacceptable particularly when the pieces of lumber have already been planed.
Moreover, it sometimes happens that in a given layer of lumber there are one or more crooked pieces which project above the upper surface of the remainder of the uppermost layer of lumber of a stack which has already been partially formed. Under these conditions the arms which lift and advance each layer to the stacking location strike against such crooked pieces of lumber, with the latter interfering undesirably with the movement of the arms, creating problems in the operation of the conventional apparatus.
Furthermore, with conventional methods and apparatus of the above type, the speed with which a given stack can be formed is limited by the time required for a pair of lumber-transporting arms to move from the lifting location to the stacking location and then back to the lifting location to raise the next layer of lumber. Such a cycle of operation of the arms which lift and transport the lumber to the stacking location requires a certain amount of time which is greater than the amount of time required by a conveyer to gather together a layer of lumber in a position ready to be transported to a stack which is being formed. As a result the full potential output of such a conveyor cannot be utilized and the time required for forming a stack of lumber is undesirably great.